MAN B&W Controllable pitch propeller. Cavitation and its types
Cavitation is associated with generation of bubbles caused by a decrease in the local pressure below the prevailing saturation pressure. The low pressure can be located at different positions on the blade as well as in the trailing wake.
When water passes the surface of the propeller it will experience areas where the pressure is below the saturation pressure eventually leading to generation of air bubbles. Further down stream the bubbles will enter a higher pressure region where the bubbles will collapse and cause noise and vibration to occur, in particular if the collapse of bubbles takes place on the hull surface.
Three main types of cavitation exist – their nature and position on the blades can be characterized as:
1. Sheet cavitation on suction side (fig. 18)
The sheet cavitation is generated at the leading edge due to a low pressure peak in this region. If the extent of cavitation is limited and the clearance to the hull is sufficient, no severe noise/vibration will occur. In case the cavitation extends to more than half of the chord length, it might develop into cloud cavitation. Cloud cavitation often leads to cavitation erosion of the blade and should therefore be avoided. Sheet cavitation in the tip vortex which will travel down stream. If the tip vortex extends to the rudder, it may cause erosion.
2. Bubble cavitation (fig. 18)
In case the propeller is overloaded – ie the blade area is too small compared to the thrust required – the mid chord area will be covered by cavitation. This type of cavitation is generally followed by cloud cavitation which may lead to erosion. Due to this it must be avoided in the design.
3. Sheet cavitation on pressure side (fig. 18)
This type of cavitation is of the same type as the suction side sheet cavitation but the generated bubbles have a tendency to collapse on the blade surface before leaving the trailing edge. The danger of erosion is eminent and the blade should therefore be designed without any pressure side cavitation.
By using advanced computer programmes the propeller designs supplied by MAN B&W Alpha will be checked for the above cavitation types and designed to minimize the extent of cavitation as well as avoiding harmful cavitation erosion.
Fig. 18. Types of cavitation
For each condition and all angular positions behind the actual hull, the flow around the blade is calculated. The extent of cavitation is evaluated with respect to noise and vibration, fig. 19.
Fig. 19. Cavitation chart and extension of sheet cavitation – suction side